SU-E-T-497: Study of Restricted Mass Stopping Power Ratio in Small Electron Tube Fields.

PURPOSE Electron tubes with small radii are useful to treat narrow regions which cannot accommodate normal electron applicators. In small electron fields, it is not trivial to estimate restricted mass stopping power ratio (MSR), which is needed to evaluate dose from ion chamber measurement. We studied MSRs in small electron tube fields using the Monte Carlo simulation. METHODS Electron tubes with radii, 3 and 2.5 cm, were used in this study. Nominal electron energies were 6 and 9 MeV. There were two types of tubes. One has a normal cut but the other has a 45-degree cut. For the normal cut tube, percent depth dose (PDD) in water was evaluated along the center of axis (CAX) of a beam. For the 45-degree cut tube, PDD was evaluated along the vertical line from the intersection of the CAX and the phantom surface with 45-degree gantry angle. The MSRs and mean electron energies were calculated using the Monte Carlo simulation. RESULTS We found good agreement between the measured and calculated PDDs. The changes of mean energies from those in the 10×10 cm2 field at the depth of maximum dose (dmax) were very small for the normal cut electron tubes. For the 45-degree cut tubes, the changes of mean energies at dmax were less than 1 MeV. The MSRs in the normal cut tube fields were almost the same as those in the 10×10 cm2 field at the corresponding depths. The MSRs for the 45-degree cut tubes deviated from those in the 10×10 cm2 by about 1% (1.5 % at most). CONCLUSIONS We evaluated the mean energies and MSRs in small electron tube fields. The deviations of them from the values in the 10×10 cm2 were small. The maximum difference of MSR was 1.5% in 45-degree cut tube fields. This work was supported by KAKENHI (23791449), Japan Society for the Promotion of Science, and Cancer Professional Training Plan, The Ministry of Education, Culture, Sports, Science and Technology, Japan.